Modern vehicles typically have one or more diagnostic systems, generally having separate computer control modules to control various functions of the vehicle. Some examples include powertrain control module (PCM), engine control module (ECM), a transmission control module (TCM), Anti-locking brake system (ABS), and an air bag control module. The vehicle diagnostic systems often have self-diagnostic capability to detect and alert the driver of problems the vehicle may be encountering, provide data stream information containing vehicle operating conditions, and allow a technician to perform special diagnostic functions to aide in vehicle repair. When a problem is found, a diagnostic trouble code (DTC) is set within a memory of a computer provided in the vehicle. DTCs are as general or as specific as the manufacturer desires.
To facilitate vehicle communications, an auto repair technician needs a diagnostic tool. The diagnostic tool must, therefore, be connected to the vehicle's computer bus system to access and retrieve vehicle data. Scan tools are testing devices that interface with vehicle diagnostic systems to retrieve information from the various control modules. The scan tools are equipped to communicate in various communication protocols such as Controller Area Network (CAN), J1850 VPM and PWM, ISO 9141, Keyword 2000 and others. These communications protocols may be specific to the various automobile manufacturers. The scan tool helps a technician diagnose and repair the vehicle based on information the tool retrieves from the vehicle.
Scan tools may include hardware for implementing a Byte Data Link Controller (BDLC) or software for simulating such hardware. This hardware can only transmit and receive individual bytes of data, similar to a Universal Asynchronous Receiver Transmitter (UART). Every time a byte of data is received or transmitted, an interrupt signal also occurs to indicate that the BLDC is ready to send the next byte or a byte is available to be received. Therefore, all messages on a bus of the hardware are handled byte-by-byte by a processor of the scan tool. Each interrupt suspends data processing until a determination is made whether the byte of data received is, for example, the start of a new data packet, data in the packet or the end of a packet. If the byte is the end of a packet, a determination is made regarding whether the packet is for the receiving processor or another processor on the bus. Thus, the processor must handle all bus traffic, whether or not it is intended for it. These interrupts delay processing and cause a lot of processor processing capability to be used for undesired data processing.